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The macro `__cplusplus` is not defined correctly in MSVC unless building with the the `/Zc:__cplusplus` flag. Instead, it defines `_MSVC_LANG` to the specified c++ standard version number. Here we introduce `EIGEN_CPLUSPLUS` which will contain the c++ version number both for MSVC and otherwise. This simplifies checks for supported features. Also replaced most instances of standard version checking via `__cplusplus` with the existing `EIGEN_COMP_CXXVER` macro for better clarity. Fixes: #2170